Multiple output switching converters are often employed to provide multiple output voltage levels from a single transformer with the outputs consisting of a main output and at least one auxiliary output. Typically, the main output is designed to operate at a higher voltage (e.g., a higher DC voltage) than the auxiliary output. When changes in load conditions occur, multiple output converters endure output voltage variations that are detrimental to the overall operation of the converter. In switching converters, a controlling parameter, such as the duty cycle of the main power switch therein, may be regulated to either increase or decrease the voltages at the main and auxiliary outputs, concurrently. When only one output is regulated, however, all other outputs may be adversely affected with load variations. The problem is exacerbated when the load requirements at the outputs move in opposite directions, since each output demands a conflicting change in duty cycle of the main power switch to adjust the output voltage to the desired value.
Common techniques for improving output regulation of a multiple output converter include weighted sum averaging, series pass regulation and shunt regulation. Weighted sum averaging allows improvement of the output regulation of one output at the expense of the other. A series pass regulator may be used to regulate small amounts of current. As current requirements increase, the series pass regulator becomes more expensive and the power losses increase. Shunt regulators divert current through series impedances in order to vary the output voltage. The current is shunted to the ground or common side and power is dissipated. Each of these techniques offers trade-offs between the efficiency and regulation of the converter.
Another technique used in multiple output converters is called a current diverter circuit as disclosed by J. Palczynski, in "Current Diverter-A Novel Circuit to Regulate Multiple Outputs," pp. 456-462, Proceedings of APEC 1993, which is incorporated herein by reference. The current diverter circuit is basically a shunt regulator for application to multiple output converters that do not have independent regulation of the separate outputs. The current diverter circuit continuously bleeds current from the higher voltage output to the lower voltage output. This technique, although an improvement from an efficiency perspective over straight shunt or series regulators, results in a continuous power loss with a resulting degradation of the power unit efficiency. The current diverter circuit is also limited by the accuracy of regulation achievable at the outputs of the converter. Additionally, there is a practical limit to the maximum power that can be bled from one output to the other.
Multiple output converters which employ independent precision regulators for each output are able to achieve accurate regulation is of both outputs. Typically, the main output voltage is controlled by regulating the energy applied to the transformer using PWM control of the main power switch on the primary side of the transformer. The other or auxiliary output, on the other hand, is regulated by an independent means on the secondary side of the transformer. The independent means may be a magnetic amplifier or an equivalent active circuit.
A particularly troubling problem exists in this multiple output converter when a significant portion of the load on the main output decreases. When the load is removed or greatly reduced, the regulating circuit for the main output drops to a minimum modulation condition. Since the auxiliary output also receives its input from energy transferred across the transformer, the independent regulating means for the auxiliary output typically cannot compensate for the substantial decline of the regulating signal for the main output. Consequently, the auxiliary output drops out of regulation for a period of time causing an unacceptable decrease in the auxiliary output voltage. If the auxiliary output is supplying power to a computer circuit, for instance, the loss of regulation, even for a brief period of time, can have catastrophic consequences which cannot be corrected even when the load at the main output returns to an acceptable level and regulation of the auxiliary output is restored.
Accordingly, what is needed in the art is a system that maintains regulation of the output voltages for a multiple output converter even in the event of transient conditions, such as a decrease in the load at the main output, while preserving the overall efficiency of the converter.